A lot of the answers depend on what kind of yarn you're using.
If, say, your yarn is knitting up at 4 stitches = one inch (just for example) and you want your blanket 36 inches, then 4 x 36 = 144 - you'd have to cast on 144 stitches.
That may be too many stitches for 14" long straight needles to hold, so you may find it easier to use 24" circular needles. You can still knit flat with circular needles, it just makes it easier to hold so many stitches.
Mirl56 at Ravelry, too.
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